1. Field of the Invention
The present invention relates generally to exposure control apparatus and, more particularly, to computer controlled photographic exposure control apparatus.
2. Description Relative to the Prior Art
One type of computer, a microcomputer, can be used for exposure control in a photographic camera by counting the cycles of its own clock to determine when to enable and/or disable particular camera functions. However, unless the clock frequency is tightly controlled by design, for example by incorporating a crystal controlled oscillator, which is expensive, the clock frequency will have a large initial variability about its nominal value. This variability requires that provisions be made to adjust the clock frequency at some step in the manufacturing process, which is also expensive.
As an alternative to establishing a predetermined clock frequency, it is known in the machine control art to use a computer (1) to measure the actual machine output using a standard reference as the machine input; (2) to calculate a correction factor for the machine that will provide the same response as that of a machine with a desired predetermined output; and (3) to store the correction factor and apply it to the machine outputs obtained using sample materials as inputs. For example, U.S. Pat. No. 3,646,331 discloses a spectrophotometer in which the actual output reflected or transmitted from a standard reference material is digitized. A correction factor is then calculated by a computer such that the digitized output multiplied by the correction factor will provide the ideal theoretical response of the spectrophotometer. The correction factor is stored, sample materials are measured and the spectrophotometer outputs are multiplied in the computer by the correction factor. One disadvantage of this approach is that as the operating conditions of the machine change, for example as the spectrophotometer lamp output changes with age, the ambient temperature changes, or the power supply varies, new correction factors must be calculated using the standard reference material as the machine input. This is not particularly undesirable in the case of a spectrophotometer, but in a photographic camera using a microcomputer for exposure control, a similar approach would require incorporating a stable, standard light source of known intensity in the camera itself. The requirement for a built-in, standardized light source is very costly and prohibits the use of the aforementioned technique in cost competitive photographic applications such as the amateur camera market.